Ending three weeks of silence, Middleboro selectmen have agreed to send a letter to Mashpee Wampanoag leaders asking them to explain published reports about the tribe's courting of Fall River for an Indian casino. In a 3-1 vote Monday, with one selectman absent, the board approved sending the letter addressed to tribal council Chairman Cedric Cromwell. The letter seeks assurance that the tribe intends to honor its contract to build a casino on 539 acres in Middleboro.